This invention relates generally to methods and apparatus for removing contaminants from aqueous systems, and more specifically relates to methods and filtration devices for removing undesired contaminants from bilgewater discharge.
Over the past two decades, U.S. regulations dealing with oily bilgewater discharge for all types of vessels have grown increasingly stringent. Even the presence of a visible oil sheen is deemed objectionable under the Uniform National Discharge Standards (xe2x80x9cUNDSxe2x80x9d). Environmental technology has struggled to keep pace, but until recently no removal method has been capable of eliminating sheen and extracting harmful contaminants from bilgewater.
The Federal Water Pollution Actxe2x80x94also known as the Clean Water Actxe2x80x94accordingly proscribes even the appearance of a visible sheen on the water, punishable by a $5,000 penalty. More specifically, the act xe2x80x9cprohibits the discharge of oil or oily waste into or upon the navigable waters of the United States or the waters of the contiguous zone if such discharge causes a film or sheen upon, or causes a sludge or emulsion beneath the surface of the water.xe2x80x9d
Further, under Sections 4301 (a) and (c) of the Oil Pollution Act of 1990, the fine for failing to notify the appropriate federal agency of a discharge has increased from a maximum of $10,000 to a maximum of $250,000 for an individual and $500,000 for an organization. The discharge of oil regulation, or xe2x80x9csheen rule,xe2x80x9d establishes the following criteria for determining a harmful oil spill:
Discharges that cause a sheen or discoloration on the surface of a body of water;
Discharges that violate applicable water quality standards; or
Discharges that cause a sludge or emulsion to be deposited beneath the surface of the water or on adjoining shorelines.
The sheen rule applies to both petroleum and non-petroleum oilsxe2x80x94e.g., vegetable oil.
The discharge regulations also have been toughened for U.S. military vessels. The National Defense Authorization Act of 1996 amended Section 312 of the Federal Water Pollution Control Act to require the Secretary of Defense and the administrator of the U.S. Environmental Protection Agency (EPA) to develop UNDS for vessels of the armed forces for xe2x80x9cdischarges, other than sewage, incidental to normal operationxe2x80x9d. Previously, this section only addressed the regulation of sewage. In consequence U.S. military vessels are limited to oily bilge water discharge concentrations of 15 ppm under 40 C.F.R. part 1700, uniform national discharge standards (UNDS).
From an environmental perspective, the increased regulatory activity in bilgewater discharge limits is warranted. The cumulative effect of vessels ranging from small recreational boats to large surface ships dumping even small amounts of bilgewater could wreak damage upon fragile aquatic ecosystemsxe2x80x94and likely has already done so.
According to nature of discharge (NOD) reports obtained from the U.S. Navy, the composition of untreated bilgewater is a varying assortment of oil and grease, oxygen-demanding substances, and organic and inorganic materials. These materials, the reports say, include volatile organic compounds, semi-volatile organic compounds, inorganic salts and metals. Oily bilge water indeed contains bio-accumulative persistent organic pollutants (POP""s) such as polyaromatic hydrocarbons and chlorinated aromatic hydrocarbons, aromatic hydrocarbons (BTEX) and oil, copper, iron, mercury, zinc and nickel, in addition to emulsifying agents such as detergents and solvents. The common metals collected in bilgewater samples include arsenic, copper, cadmium, chromium, lead, mercury, selenium and zinc, while organics include benzene, isomers of hexachlorocyclohexane, ethyl benzene, heptachlor, heptachlor expoxide, napthalene, phenols, pthalate esters, toluene, trichlorobenzene and trichloroethane.
The primary sources of these contaminants are vessel propulsion systems and auxiliary systems that use fuels, lubricants, hydraulic fluid, antifreeze, solvents and cleaning chemicals. Certain waste streams such as steam condensate, boiler blowdown, drinking fountain water, and sink drainage located in various machinery spaces can also drain to the bilge.
In worst-case scenarios, environmentally irresponsible vessel operators have dealt with excess bilgewater by dumping it overboard. Others have tried to hide the telltale sheen with emulsifiers, though the damage is still done.
Currently, commercial and military surface ships primarily employ two different methods in dealing with bilgewater treatment and removal. Many of these vessels use oil water separator (OWS) systems to reduce the oil content of bilgewater prior to overboard discharge. Most of these large vessels also have an onboard systems for collecting and transferring bilgewater to a holding tank for later removal and disposal on shore.
In general, OWS technology is unable to meet the 15 ppm requirement. Many OWS systems on cruise and naval ships produce an effluent in the average range of 100-250 ppm. A ship equipped with an oil content monitor (OCM) has the ability to return bilge water, not meeting discharge standards to the OWS for reprocessing. The effluent concentration after OWS is more a measure of the degree of emulsification of the influent than of the efficiency of the OWS. With 100% non-aqueous phase pollutants, OWS are capable of achieving quite low effluent concentrations. All OWS systems operate on the principle of gravity separation driven by density differences. If the suspended particles or droplets have effectively neutral buoyancy, OWS ceases to be effective. Additionally, OWS systems are ineffective in removing colloidal metals and soluble compounds. By definition, these are close to or at neutral buoyancy. Accordingly there is a need to look for other properties to exploit in order to achieve the desired reduction of pollutants in bilge water.
In accordance with one aspect of the present invention, it has been found that the compositions disclosed in the present inventor""s U.S. Pat. Nos. 5,437,793; 5,698,139; 5,837,146; and 5,961,823 (all of which disclosures are hereby incorporated by reference) have extremely strong affinities for the aforementioned contaminants in oily bilgewater; and that when oily bilgewater streams containing these noxious contaminants are passed through filtration media incorporating these inventive compositions, the contaminants are immobilized at the media, as a result of which concentration levels of the contaminants in the filtrate may be reduced to very low values, in some instances below detectable limits in a single pass. Use of the invention not only enables ready removal from the bilgewater of oils, greases and the like, but as well removal of pernicious slightly soluble organic compounds such as benzene, toluene, xylene, halogenated hydrocarbons, ethoxylated glycols, etc. These noxious contaminants are among the more difficult compounds to remove from water, and indeed most are carcinogenic. The solubility of the foregoing substances renders most prior art physical separation methods ineffective and causes formation of stable and pseudostable oily emulsions (miscelle size of 400 micrometers or less) which also don""t respond well to gravity separation due to neutral buoyancy. Among the metal ions which additionally may be removable by the methods and apparatus of the invention are those of cadmium, chromium, copper, lead, nickel, zinc, arsenic, silver, and mercury.
In accordance with one aspect of the present invention, the oily bilgewater is passed through one or more filters incorporating the principles of the invention, prior to the bilgewater being actually discharged from the vessel. In this arrangement the filter or filters may simply be placed directly in the bilgewater discharge line, e.g. downstream of the bilgewater pump effecting the discharge flow. In a test of a typical such installation, and using a dwell time of one second and a flow rate of 20 gallons per minute, a filter in accordance with the invention reduced all contaminants to below detectable limits (BDL) at various percentages of oil contamination. The test results included removal of all sheen and visible discharge. The methods of the invention have the ability to remove from the bilgewater most mixed emulsions and lipohilic and hydrophilic organic compounds, in addition to many chelated and colloidal metals.
In another aspect of the invention; filtration media incorporating the applicant""s aforementioned compositions, may be used in conjunction with cross current flow membranes, to reduce or eliminate the known tendency of such membranes to rapidly become fouled and inefficient when used to filter bilgewater. In this arrangement the filters of the invention can be placed upstream of the membrane filter. The ability of the inventive filters to absorb concentrated slugs without breakthrough or significant pressure drop makes them ideal chemical prefilters to work in conjunction with membrane systems.
Almost half the cost of wastewater filtration utilizing membrane technologies (reverse osmosis, RO) arises from clearing clogged and fouled filtration membranes (Business Week 2/26/01). These membranes are composed of water repellant (hydrophobic), oil attracting polymers. Hydrophilic polymer membranes generally do not have the required mechanical strength for membrane filtration applications. Hydrophobic polymeric membranes are clogged easily by medium to high molecular weight proteins and oily oleophilic molecules that are suspended in water. Membrane clogging makes it necessary to use anti-clogging chemical additives. In spite of this, frequent maintenance and cleaning of the membranes is required. Chemical cleaning additives tend to denature the membrane making replacement necessary. Membranes are very expensive making this an undesirable scenario. Experimental attempts have been made to dope hydrophobic membranes with hydrophilic polymers during membrane fabrication. Although a ten fold performance improvement is anticipated, the results are experimental and doping with hydrophilic polymers could lead to unanticipated problems such as increased scaling due to inorganic pollutants which are also hydrophilic. Oleophilic pre-filters in accordance with the invention are capable of removing 100% of high molecular weight and non-aqueous phase organic compounds in one pass with close to zero pressure increase across the filter to saturation. Pre-filtration with these units eliminates organic fouling of membrane filtration devices resulting in 50% decrease in operating cost, twenty fold increase in efficiency and ten fold reduction of filter turnover.
Filter configurations incorporating the applicant""s above described compositions (hereinafter referred to as xe2x80x9cabsorbent compositionsxe2x80x9d) may be based on various water permeable substrates, such as shredded, spun or otherwise configured polypropylene or shredded or spun cellulose, which substrates are infused or otherwise treated with the absorbent compositions, which are then cured. These substrates may then be packed or otherwise disposed in a cartridge or canister filter; or can be formed into cured and infused bag filters which can be emplaced in canisters through which the contaminated bilgewater is flowed. Similarly the said absorbent compositions can be incorporated into or upon other filtering substrates and media, such as paper, including compressed pulp materials, particulate porous foamed plastics, mineral particulates such as perlite and vermiculite, and particulate, fibrous or porous ceramic or porous (e.g. sintered) metal substrates and media. For purposes of the present specification the term xe2x80x9cchemical affinity separationxe2x80x9d may from time to time be used to refer to the use of otherwise conventional filter materials which have been infused with curable polymeric surfactant (herein xe2x80x9cPSxe2x80x9d) as discussed in the above referenced patents. The resultant filter will for convenience be referred to as a xe2x80x9cPST filterxe2x80x9d, i.e., a filter based on xe2x80x9cpolymeric surfactant technologyxe2x80x9d. Once cured into a substrate, the oleophilic properties of PS are transferred into a substrate thereby greatly enhancing its ability to attach organic compounds to its matrix. Once attached, these compounds become hydrophilic and tend not to re-release.